This invention relates generally to electronic pacer devices for providing artificial electrical stimulation to the heart of a patient and more specifically to an improved pacer of the demand-inhibit type which incorporates digital logic circuitry in the pulse generator for providing several unique features not presently found in commercially available pacers.
In the normal heart, electrical signals are generated and appear in the atrium at a rate of approximately 60 to 120 times per minute, depending upon the physical activity of the individual. About 0.1 seconds following the generation of each atrial signal, a signal is transmitted to the ventricle, causing it to contract and force blood from the ventricle through the body. Following the ventricular contraction, the heart experiences a refractory delay period which persists approximately 0.4 seconds and during which the heart is unresponsive to electrical stimulation.
An often observed abnormality of the heart is its failure to regularly or periodically generate atrial signals. Atrial signals may be generated with perfect regularity but then, suddenly, cease altogether or occur at an abnormally low rate. To compensate for such a defect the so-called "demand-inhibit" pacer is a practical solution. Such a device applies stimulating pulses to the ventricle by means of surgically implanted electrodes only when natural pulses do not occur spontaneously. As long as natural pulses occur at a desired rate, the implanted pacer provides no stimulation. However, when the natural or spontaneous ventricual pulses fail to occur or occur at irregular intervals the pacer comes into play to provide artificial stimulation at a desired rate.
Prior art heart pacers as well as the pacer of the present invention include a simulated refractory delay period. The reason for including a simulated refractory delay in the pacer circuitry is to ignore spontaneous signals developed by the heart which occur within a prescribed time period following application of an artificial stimulating pulse.
The pacer of the present invention operates asynchronously at a predetermined frequency (e.g., 100 beats per minute) when a permanent magnet is applied over the implanted pacer. This is the so-called "magnetic rate". The unit will remain in the magnetic rate mode as long as the permanent magnet is in proximity to the implanted pacer device and for a maximum of 2,256 milliseconds after removal of the magnet. Immediately after the magnetic rate interval, the pacer will operate asynchronously at the programmed nominal rate for six pacer pulses. This allows the pulse generator to be checked for both magnetic ate and nominal rate with the use of an externally applied magnetic field.
Another aspect of the present invention is the incorporation of an Elective Replacement Time indicator feature. The ERT indicator is tied in with the magnet rate of 100 beats per minute, therefore, a magnet is also used to check the ERT. When the output of the implanted energy source (battery) has decreased 1/2 of the way to ERT, the magnetic rate will drop to 90 beats per minute. When the ERT is reached, the magnetic rate will drop to 85 beats per minute. Therefore, a technician can readily determine by sensing the frequency or rate of pacer pulses when in the magnetic rate mode just when further surgery may be required to replace the energy source.
The pacer circuit of the present invention also includes noise protecting circuitry. Specifically, a bandpass amplifier is used as a signal discriminator which attenuated unwanted signals, i.e., EMI, T-waves, 60 cycle pickup, etc., but passes signals containing the fundamental frequency component of R-waves. Thus, all unwanted signals are filtered and only desirable signals are passed. Noise protection is afforded by the fact that the pacer of the present invention includes apparatus for monitoring the pacer inputs during refractory intervals. If for any reason, electrical noise should pass through the bandpass amplifier, a noise detection circuit, including a chain of bistable flip-flops, takes over and switches the pulse generator into its asynchronous mode in which state it remains until the noise source is no longer present.
Another feature of the pacer of the present invention is the incorporation of a dual refractory system. More specifically, the circuitry incorporated in the pacer provides a 256 millisecond refractory period following the occurrence of a natural heartbeat and a 320 millisecond refractory period following a heartbeat that is a result of an artificial stimulating pacer pulse. During the final 64 milliseconds of the refractory periods, the pulse generator will be monitoring the heart electrodes for electrical signal activity, but will not allow the resetting of the pulse generator timing. This too is important in rendering the device immune from external noise.
The pacer system of the present invention also permits reprogramming of the rate of generation of pacer pulses as well as the width of such pulses. Programming is accomplished by means of an external console into which an operator may enter a code combination which functions to unlock circuitry in the implanted unit to allow new pulse rate and pulse width information to be entered. This is accomplished by means of an electromagnet which is positioned outside of the body but in proximity to the implanted pacer unit. The electromagnet is pulsed by the control circuits in the console and operates a magnetic reed switch which is found in the implanted circuitry. A three digit pulse train is employed to allow programming of the pulse generator. Before a new pulse rate may be entered, the external programming unit must first present a certain preselected code, such as a 5-3-4 code to the reed switch. Similarly, before the width of the pacer pulse may be changed, a second preselected code, such as a 5-3-2 code must be entered.
Upon application of the first pulse of the initial digit (5) the circuitry will revert to its magnetic rate mode (100 BPM) and will remain there until the programming operation has been completed. When programming in a new pulse rate, the operator initially presents a 5-3-4 address code to the pulse generator. Once the pulse rate generator is addressed, a single pulse from the programmer initiates the pulse period. A second pulse from the programmer terminates the pulse period at the desired pulse interval. This interval, in conjunction with the pulse generator internal clock, allows the programming of any rate between 30 and 120 BPM. The programming time for any rate between 30 and 120 BPM is 2,256 milliseconds. Regardless of the rate programmed, i.e. between 30 and 120 BPM, the pulse generator will remain in the magnetic rate for the full 2,256 milliseconds.
Should an abnormally long or short pulse rate be supplied to the pulse generator, the following will occur. In the case of an excessively long rate, e.g., greater than 2,048 milliseconds, the pulse generator will remain in the paced magnetic rate mode (100 BPM) until an acceptable rate is inserted. If the rate is less than 496 milliseconds, the pulse generator will remain in the paced magnetic rate mode until an acceptable rate is programmed.
An alternate width for the pacer pulse is programmed in a somewhat different manner. Once the pulse generator is addressed, a pulse train corresponding to a particular pulse width is presented to the pulse generator. The minimum width is assured by a hard wired connection within the pacer unit and is 0.05 milliseconds. This is incorporated to guard against failure to enter any width count after successfully addressing the pulse generator by the 5-3-2 code. In the event that an excessive number of pulses are included in the pulse train, the pulse generator will reach its maximum width of 3.2 milliseconds and recycle back to 0.10 milliseconds. It will continue to cycle until no more pulses are received. Following the reprogramming operation, the pulse generator switches to its asynchronous mode for six pacer pulses and then to the demand mode.
The three digits of the unlocking codes are transmitted in three consecutive 68 milliseconds time frames. The first digit time frame overlaps the pulse generator's first digit time frame by 4 milliseconds to ensure entry of the digit. The second digit time frame overlaps the pulse generator's second digit time frame by 8 milliseconds and the third digit overlaps by 12 milliseconds. This overlap will ensure that the unlocking code is entered even if the clock frequency in the implanted unit has drifted somewhat from its design value.